As a printer, for example, for printing receipts for cash registers or a portable label printer for printing POS labels for foods and labels for logistics management, a thermal printer is frequently employed wherein a thermal head that includes a heat generating member is pressed against a platen roller, and thermal recording paper is sandwiched between them to perform printing.
According to the structure of a conventional thermal printer, for example, provided at the least are printing means, which includes a thermal head, conveying means, which includes a platen roller, and a drive transmission mechanism, which includes a motor that serves as the drive source for the platen roller, that are arranged in a main frame, and the main frame wherein these members are mounted is fixed at a predetermined position in a printer housing.
FIG. 5 is a perspective view of the drive transmission mechanism of a conventional thermal printer. In FIG. 5, a main frame 30 has substantially a half-rectangular shape consisting of side-walls, opposite the direction of the paper width, and a bottom plate. An insertion hole 30a is formed in a right side wall 30R in order to insert a drive gear 21a of a motor 21 and to position the motor 21, and beside the insertion hole 30a, two gear support shafts 31, 32 are placed so as to project outward. Since appropriate rigidity and durability are required for the main frame 30, generally, sheet metal processing is performed using a metallic material, such as steel, to obtain the main frame 30 having a predetermined shape. Therefore, it is difficult to integrally form the gear support shafts 31, 32, with the main frame 30, and as described above, the gear support shafts 31, 32 are arranged by forced insertion.
Further, idler gears 23, 24, which transmit the drive force from the motor 21 to the platen roller, are fitted over the gear support shafts 31, 32 of the main frame 30, and the motor 21 is attached to the main frame 30 by screws and engages the idler gears 23, 24. The individual parts, such as the idler gears 23, 24 and the motor 21, that are mounted on the main frame 30 in this manner constitute the drive transmission mechanism. Further, there is another thermal printer wherein, although not shown in FIG. 5, a dust cover is provided outside gears to prevent dust from entering the drive transmission mechanism.
Furthermore, as an additional technique related to a drive transmission mechanism for driving a platen roller, the present inventor has proposed thermal printers wherein a housing frame is constituted as a resin frame separable into two upper and lower segments, and wherein a plurality of holding units that freely hold the rotary shafts of a motor and idler gears are formed in the housing frame (see patent documents 1 and 2).    Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-237118    Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-237121
In recent years, a reduction in the sizes of parts, such as gears, has been requested in order to downsize thermal printers, and accordingly, the fitting accuracy for the parts must be more improved. For example, a tolerance for the drive transmission mechanism must be 30 μm or less. This is because when variations in part manufacturing or variations in assembly are large, the inter-shaft distances among the drive gear of the motor, the idler gear and the coupled gear of the platen roller are not fixed, and gear engagement errors at the drive transmission mechanism occur, etc., and cause deterioration of the efficiency of motor torque.
According to the structure of the thermal printer in FIG. 5, the idler gears 23, 24 are fitted over the gear support shafts 31, 32 forcibly inserted into the main frame 30, and engage the drive gear 21a of the motor 21 and the coupled gear of the platen roller. Therefore, many factors are present that adversely affect the tolerance between gears, and accurate assembly of the drive transmission mechanism is limited. That is, the tolerances for the gears is affected by an inclination that occurs when the gear support shafts 31, 32 are forcibly inserted, and also, since the motor 21 is positioned by the insertion hole 30a in the main frame 30, it is not easy to mount the motor 21 very accurately. Therefore, problems would arise in that efficiency of transmission of motor torque is deteriorated and in that durability of the drive transmission mechanism is reduced.
In addition, according to the conventional technique, assembly of the drive transmission mechanism is simplified; however, since a resin frame is employed, durability and heat releasing properties are inferior to those of a steel frame. Therefore, in order to cope with downsizing and increasing output, it can not always be said that this technique is appropriate.
One objective of the present invention is to provide a printer that employs a platen roller as conveying means, for which the accuracy of the assembly of the constituents of a drive transmission mechanism that drives a platen roller is increased, and the heat releasing function of a motor that is a drive source is also improved, so that downsizing and increasing output can be coped with.